How do you figure out the dimensions of 800 sq ft?

There's no way. There are an infinite number of different shapes with different dimensions
that all have areas of 800 sq ft.

If it's a circle, then the diameter is about 31.9 ft.

If it's a square, then each side is about 28.3 ft long.

If it's a rectangle, then it could be (80 ft by 10 ft), or (40 ft by 20 ft), or (160 ft by 5 ft), or (50 ft by 16 ft),
or an infinite number of other rectangles.